The present invention relates to a fixing device for use in electrophotographic devices.
In electrophotographic devices, which are applied to copiers, printers, facsimiles and complex machines composed thereof, toners are fixed onto recording paper by transferring toner images on the recording paper and heating the recording paper while keeping the recording paper in between a heating body (heating roller or heating belt) and a pressing body (pressure roller or pressure belt).
In such fixing process, sufficient adhesive strength of toners to the recording paper is sometimes not achieved simply by heating a printing surface onto which toner images were transferred with a heating roller (or heating belt). Therefore, a heat source such as halogen lamps is generally placed inside each of the heating roller and the pressure roller (or heating belt).
As disclosed in JP H10-162944 A, U.S. Pat. No. 6,341,211 and JP 2003-317923 A, a technology for induction heating conducted by applying alternating magnetic fields to the heating roller is publicly known.
The induction heating requires placement of expensive exciting coils for applying alternating magnetic fields to heating elements, and placing the exciting coil in each of the heating roller and the pressure roller has imposed a heavy burden in terms of costs.
The technology disclosed in JP H10-162944 A is to provide an exciting coil extending through both a heating roller and a pressure roller so as to heat both the heating roller and the pressure roller with a single exciting coil. However, winding the exciting coil so as to alternatively go through the heating roller and the pressure roller is technically difficult to achieve and may adversely cause cost increase.
In U.S. Pat. No. 6,341,211, the frequency of a drive voltage applied to an exciting coil is controlled so as to allow downsizing of a closed magnetic circuit core, though the aspect of heating the pressure roller is not at all disclosed.
The technology disclosed in JP 2003-317923 A is to divide an exciting coil into a plurality of parts so as to constitute circuits having different resonance frequencies by resonant capacitors and to select an exciting coil to function by frequencies of drive voltages. However, this technology is to make a heating value of the heating roller appropriate and the aspect of heating the pressure roller is not at all disclosed.